This invention is related to a two-tube shock absorber with controllable variable shock absorbing power, and more particularly to a two-tube shock absorber for automotive vehicles, that includes a power cylinder whose interior space is subdivided by means of a piston being slidable by a piston rod into a first and a second power chamber; a storage tank, being partly filled with oil, with hydraulic connections which provides a balance of volume between the power chambers and the storage tank; a first and a second non-return valve; and an electromagnetically actuatable shock absorber valve for the variation of the cross-sectional area of flow of one of the hydraulic connections.
A two-tube shock absorber is known from the international patent application, No. WO 89/09891. The particular feature of that two-tube shock absorber consists in that its shock absorbing power is predetermined exclusively by means of a two-stage controllable shock absorber valve which is positioned at the lower end of the central tube and through which the flow is passed in one direction only. In that configuration, the pilot control stage of the shock absorber valve serves exclusively for the actuation of the main stage whose switching condition determines the variable shock absorbing power.
However, the control technique incorporated in the prior-art shock absorber includes the drawback based on the circumstance that the electric control signal for the actuation of the shock absorber valve is proportional to the desired shock absorbing power. This results from the application of so-called overpressure valves leading to a family of characteristic curves that do not encompass the range of operation required for a semiactive shock absorber. It is, furthermore, considered another disadvantage that in the event of each zero passage of the piston speed a sudden change of the shock absorbing power takes place. In addition, it is of disadvantage that for the generation of a predetermined desired characteristic curve, during a shock absorber stroke, the actuation of the pilot control stage continually must be changed. This process is highly time-dependent and can, therefore, be realized only with elevated design efforts. This circumstance leads to the fact that each time delay in the actuation of the valve unfavorably affects the chassis control process.
It is, therefore, an object of the present invention to provide a two-tube shock absorber that overcomes the aforementioned disadvantages. Moreover, a two-tube shock absorber, in accordance with this invention, affords an adjustment of a special desired characteristic curve; of the dependence of the shock absorbing power on the piston speed. The desired characteristic can be chosen out of a family of potential characteristic curves which includes progressive and degressive characteristics. Simultaneously, the sensitivity of the shock absorber with respect to actuating errors is to be eliminated.